Periodic Activity of Metals Periodic Trends and the Properties of the Elements SCIENTIFIC Introduction Elements are classified based on similarities, differences, and trends in their properties, including their chemical reactions. The reactions of alkali and alkaline earth metals with water are pretty spectacular chemical reactions. Mixtures bubble and boil, fizz and hiss, and may even smoke and burn. Introduce the study of the periodic table and periodic trends with this exciting demonstration of the activity of metals. Concepts • Alkali and alkaline earth metals • Periodic table and trends • Physical and chemical properties • Metal activity Materials Calcium turnings, Ca, 0.3 g Beaker, Berzelius (tall-form), Pyrex®, 500-mL, 4 Lithium metal, Li, precut piece Forceps or tongs Magnesium ribbon, Mg, 3-cm Knife (optional) Sodium metal, Na, precut piece Petri dishes, disposable, 4 Phenolphthalein, 1% solution, 2 mL Scissors Water, distilled or deionized, 600 mL Safety Precautions Lithium and sodium are flammable, water-reactive, corrosive solids; dangerous when exposed to heat or flame. They react violently with water to produce flammable hydrogen gas and solutions of corrosive metal hydroxides. Hydrogen gas may be released in sufficient quantities to cause ignition. Do NOT “scale up” this demonstration using larger pieces of sodium or lithium! These metals are shipped in dry mineral oil. Store them in mineral oil until immediately before use. Do not allow these metals to stand exposed to air from one class period to another or for extended periods of time. Purchasing small, pre-cut pieces of lithium and sodium greatly reduces their potential hazard. Calcium metal is flammable in finely divided form and reacts upon contact with water to give flammable hydrogen gas and corrosive calcium hydroxide. Magnesium metal is a flammable solid and burns with an intense flame. Perform this demonstration in a well-ventilated lab only. Do not handle any of the metals with bare hands. Wear chemical splash goggles, chemical- resistant gloves, and a chemical-resistant apron. All students or spectators should also be wearing chemical splash goggles during this demonstration. Use a Class D powder fire extinguisher such as dry sand for reactive metals. Please review current Material Safety Data Sheets for safety, handling, and disposal information. Preparation 1. Obtain four 500-mL, tall-form beakers and label them Li, Na, Mg, and Ca. Add approximately 150 mL of distilled or deionized water to each beaker. Label four Petri dishes Li, Na, Mg, and Ca and place them next to the beakers. 2. Cut the magnesium ribbon into 3-cm strips using scissors. 3. The precut lithium and sodium metal pieces should be approximately 0.5 cm × 0.5 cm × 0.5 cm, or 0.2–0.3 g each. 4. Divide the calcium turnings into five samples, about 0.3 g each. Procedure 1. Place one piece of each metal in its respective Petri dish on an overhead projector. Observe and compare the physical properties of the metals: Color, luster (shine), hardness, and malleability. 2. Have students record the properties of the elements on the worksheet. © 2016 Flinn Scientific, Inc. All Rights Reserved. Publication No. 95022 1 061616 Periodic Activity of Metals continued 3. Discuss possible “rankings” of the metals with respect to their physical properties. Which metal appears to be the shiniest? Softest (or hardest)? 4. Use forceps or tongs to quickly transfer one piece of sodium metal to water in its respective labeled (Na) beaker. Have students make detailed observations of the resulting chemical reaction and record all observations on the worksheet. Sodium metal, which initially floats on the surface, immediately begins to “hiss and sizzle” on the water surface and appears to melt. Popping sounds are heard and the metal begins to bounce around and finally disappear. A smoky gas forms and ignites the metal on the surface of the water. Sparks may be seen where the metal ignites. 5. Repeat step 4 using lithium metal in its respective (Li) beaker. As observations are made, ask students to compare the rate and intensity of the reaction versus that of sodium. Record all observations on the worksheet. 6. Repeat step 4 twice more, using calcium and magnesium, respectively. Compare the activity of each metal against the previous metal and against sodium as a reference metal. Record all observations 7. Refer to the observations to rank the metals in order of their reactivity. Which metal is most active? Least active? Answer Questions #1 and #2 on the worksheet. 8. Refer to Question #3 on the worksheet. Based on class discussion, draw arrows to indicate the directions in which metal activity increases across a row and within a column in the periodic table, as shown below. Activity increases Li Na Mg Ca 9. Ask students to predict the activity of potassium metal based on the observed periodic trend in the activity of metals (Answer Question #4 on the worksheet). Discuss the extreme reactivity of potassium metal (and why it was not used in this demonstration). 10. After the metals have reacted (with the exception of magnesium, which does not react under these conditions), add 5 drops of phenolphthalein solution to the mixture in each beaker. 11. Observe the color change(s) and discuss what a color change indicates. What types of solutions exhibit this color change? Discuss the possible identity of the product(s). (See the Discussion section.) 12. Write balanced chemical equations for the reactions of the metals with water (Question #5). Discuss the evidence for the formation of both hydrogen gas and metal hydroxides. 13. (Optional) Have students write a paragraph describing in words the physical and chemical properties of one of the metals. Instruct students to include as much descriptive detail as possible. An example is given below. “Sodium is a soft, silver-white solid. Upon exposure to air it gradually develops a white oxide coating. It can be cut with a knife. It is less dense than water and reacts spontaneously and vigorously on contact with water. The metal piece appears to pop or sizzle on the surface and a smoky white gas forms. The metal may ignite on the surface of the water in the vicinity of the smoke. The products are hydrogen gas and sodium hydroxide. The hydrogen gas that is formed ‘pops’ and briefly ignites. Sodium hydroxide makes the solution basic (red) to phenolphthalein indicator.” Disposal Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures governing the disposal of laboratory waste. Use tongs or forceps to remove unreacted magnesium from its beaker. Dispose of excess magnesium metal according to Flinn Suggested Disposal Method #26a. Do not dispose of any of the other reaction mixtures until all of the metal in each has completely reacted. The resulting basic solutions in each beaker can be neutralized and disposed of according to Flinn Suggested Disposal Method #10. 2 © 2016 Flinn Scientific, Inc. All Rights Reserved. Periodic Activity of Metals continued Tips • We strongly recommend the purchase of small quantities only of sodium and lithium metal. Purchase only the amounts that will be used in one academic year. Sodium and lithium are available in small, precut pieces that are suitable for demonstrations (Flinn Catalog Nos. S0329 and L0057, respectively). • We do not recommend the use of potassium in the high school science laboratory. Potassium is considerably more water- reactive than sodium and is a serious fire and explosion hazard. There is a significant and often undetectable explosion risk because of the propensity of potassium to form a superoxide (peroxide) coating on its surface. Potassium reacts with oxygen in air to form a coating of yellow potassium superoxide (KO2), even when the metal is stored under dry mineral oil. Old pieces of potassium are thus extremely dangerous. When the metal is cut, the pressure of the knife may touch off a violent, uncontrollable, explosive reaction between the superoxide coating and the underlying metal. • Calcium metal must be reasonably fresh to react with water. Old (dull) calcium metal will not react with water. • The reactions of sodium and lithium with water may be quite vigorous—we recommend using tall-form (Berzelius) beakers to contain any molten metal pieces that may splatter. Do NOT scale up this demonstration. • The use of a ChemCam™ video camera will make it easier for students to observe the appearance and properties of the metal pieces. • Demonstrate the softness of lithium and sodium by showing how the metal pieces can be cut with a dry spatula or knife. • In ranking the metals in order of their activity, it is easier to begin with pairwise comparisons. Which is more active— sodium or lithium? Calcium or magnesium? Magnesium or sodium? • Is the activity of metals related to their hardness? Density? The answer, a firm “maybe,” depends on the comparisons being made. The alkali metals as a group are softer and less dense than their nearest alkaline earth metal neighbors, and also more reactive. Within the group of alkali metals, however, the opposite trend is observed. Lithium is less dense but also less reactive than sodium. Discussion Sodium reacts with water to form hydrogen gas and sodium hydroxide, according to the following balanced chemical equation. 2Na(s) + 2H2O(l) → H2(g) + 2NaOH(aq) + Heat As sodium metal reacts with water, a great deal of heat is generated. The sodium melts and seems to float or bob on the water surface. The oxide coating that may have dulled the metal disappears and sodium’s silvery gray, metallic character is more apparent. The evolution of hydrogen gas is evident in the production of a white smoke, which pops and ignites as it is heated above its flash point.